Introduce v8::Exception::GetMessage to find location of an error object.

API=v8::Exception::GetMessage
BUG=chromium:427954
R=yangguo@chromium.org
LOG=Y

Review URL: https://codereview.chromium.org/687253002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@25015 ce2b1a6d-e550-0410-aec6-3dcde31c8c00